What would happen if North Korea launched nuclear missiles toward the heartland? How would the weapons be stopped?

On a day when President Donald Trump threatened North Korea with "the fire and the fury like the world has never seen," and a week after some experts assessed that Pyongyang had missiles capable of reaching the U.S. mainland, defense officials say there are three levels of protection.

1. Ground-based interceptors on the Korean peninsula that could stop North Korean intercontinental ballistic missiles far from the U.S. mainland.

2. Sea-to-air defensive rockets on U.S. Navy ships in the region that are considered a "mid-range" solution.

3. The final line of defense against an attack by North Korea is two batteries of surface-to-air missiles based in Alaska and California.

Appearing at a golf event at his club in Bedminster, N.J., where Trump is on a 17-day "working vacation," the president Tuesday said, "North Korea best not make any more threats of the United States." His remarks were in response to news Tuesday that North Korea now has miniaturized a nuclear warhead that can be put inside a missile.

If attack missiles made it past U.S. ground and sea-based interceptors closer to North Korea -- and were headed toward middle America -- defensive positions on the west coast would be the last point of protection. According to the Pentagon, there are 36 interceptors in place at Fort Greely, Alaska, southeast of Fairbanks and Vandenberg Air Force Base in Santa Barbara County, California. The Missile Defense Integration & Operations Center that monitors attacks and controls responses is located in Colorado Springs, Colorado.

The North Koreans are now believed to have between 30 and 60 nuclear-tipped missiles.

The American interceptor has an uneven track record, having succeeded in only 50 percent of tests against missiles since 1999.

The roots of the $40 billion system are in President Ronald Reagan's efforts to develop a response to ballistic missile threats during the Cold War, when tensions were high between the U.S. and Soviet Union. It has been in place since 2004 and never used in combat or fully tested.

A rocket soars into space, then releases what is called a "kill vehicle" equipped with a system that guides it into the path of a missile. In May an interceptor launched from Vandenberg AFB targeted a simulated ICBM launched from a test range in the Pacific. The simulated missile flew faster than those used in previous ground-based interceptor tests and the exercise was considered successful.

"We need to calm down, take a deep breath and realize that the North [Korea] is not going to attack us," said Tom Collina, a nuclear policy expert. "There's a risk that they [North Korea] will make a mistake... that they will uh look at the situation in a way that is miscalculation and that we will stumble into catastrophe."